1. Field of the Invention
The present invention relates to a positive electrode active material and a method of producing the same, a positive electrode, and a non-aqueous electrolyte secondary battery. More specifically, the present invention relates to a positive electrode active material providing a non-aqueous electrolyte secondary battery having an excellent cycle characteristic, a method of producing the same, and a positive electrode and a non-aqueous electrolyte secondary battery using the same.
2. Description of the Related Art
As a non-aqueous electrolyte secondary battery, a lithium secondary battery has been put into practical use, and has been widely used. Furthermore, in recent years, a lithium secondary battery attracts attention as a large-capacity device for automobile use or electric power storage, as well as a small-size one for use in a portable electronic device. Therefore, higher requirements of safety, cost, service life and the like have been imposed.
A lithium secondary battery mainly consists of a positive electrode, an negative electrode, an electrolytic solution, a separator and an exterior material. The aforementioned positive electrode consists of a positive electrode active material, a conductive material, a collector and a binder (binding agent).
Generally, as a positive electrode active material, a layered transition metal oxide represented by lithium cobaltate (LiCoO2) is used. However, a layered transition metal oxide is susceptible to oxygen desorption at a relatively low temperature around 150° C. in a fully charged state, and a thermal runaway reaction of the battery can occur due to the oxygen desorption. Therefore, in using a battery having such a positive electrode active material in a portable electronic device, accidents such as heat generation and ignition of the battery can occur.
For this reason, a lithium-containing composite oxide having a stable structure, not emitting oxygen in an abnormal condition, and having an olivine structure that is safer than LiCoO2, for example, lithium iron phosphate (LiFePO4) is expected. Lithium iron phosphate has a merit that it is relatively low in cost because it does not contain cobalt exhibiting low crustal abundance. Lithium iron phosphate also has a merit that it has more stable structure than a layered transition metal oxide.
However, when lithium iron phosphate is used as a positive electrode active material, there arise the problems that decrease in discharge capacity due to repeated charging/discharging is large, and a service life of a battery obtained therefrom is short. This is because since expansion or contraction of a positive electrode active material caused by insertion or desorption of Li+ due to charging/discharging is large, the positive electrode active material gradually drops off physically from the collector or the conductive material due to increased number of cycles, and the structure of the positive electrode active material is broken, and the active material not contributing to charging/discharging increases to cause drop of the discharge capacity. For addressing to this problem, there has been proposed a method of preventing expansion or contraction of a positive electrode active material by using, as a positive electrode active material, a lithium-containing composite oxide obtained by subjecting a basic structure of lithium iron phosphate obtained by a solid-phase method to element substitution (for example, Japanese Unexamined Patent Publication No. 2002-198050 and Japanese Translation of PCT International Application Publication No. 2005-519451).